Process for producing zirconium oxide thin films

ABSTRACT

This invention concerns a process for producing oxide thin film on a substrate by an ALD type process. According to the process, alternating vapour-phase pulses of at least one metal source material, and at least one oxygen source material are fed into a reaction space and contacted with the substrate. According to the invention, an yttrium source material and a zirconium source material are alternately used as the metal source material so as to form an yttrium-stabilised zirconium oxide (YSZ) thin film on a substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to oxide thin films produced by an ALDmethod. In particular, the present invention relates toyttrium-stabilised zirconium oxide (YSZ) thin films.

2. Description of Related Art

The continuous decrease in the size of microelectronic components leadsto the situation in which SiO₂ used today as the gate oxide in metaloxide semiconductor field effect transitions (MOSEFT) must be replacedwith a higher permittivity oxide. This is due to the fact that in orderto achieve the required capacitances, the SiO₂ layer should be made sothin that the tunneling current would increase to a level affecting thefunctioning of the component. This problem can be solved by using adielectric material having a higher dielectric constant than SiO₂. Forexample, the capacitance of dynamic random access memory (DRAM)capacitors must remain nearly constant while their size decreasesrapidly, and thus it is necessary to replace the previously used SiO₂and Si₃N₄ with materials which have higher permittivities than these andgive higher capacitance density.

There is a number of materials exhibiting sufficiently high dielectricconstant, but in addition to high permittivities, these dielectric thinfilms are required to have, among other things, low leakage currentdensities and high dielectric breakdown fields. The achievement of bothof these properties presupposes a dense and flawless film structure. Itis also important that the materials are stable in contact with siliconand can be exposed to the high post-treatment temperatures essentiallywithout changes. Especially in the gate oxide application it isimportant that in the interface between silicon and the metal oxidehaving high dielectric constant there are very few electrically activestates. In the memory application it is important that the structure ofthe dielectric of the capacitor is stable, since the temperatures usedfor activation of implanted ions are high.

Zirconium oxide, ZrO₂ is an insulating material having a high meltingpoint and good chemical stability. ZrO₂ can be further stabilised byadding other oxides, the aim of adding other oxides is to eliminate thephase changes of ZrO₂. Normally, the monoclinic crystal form is stableup to 1100° C. and tetragonal up to 2285° C., above which the cubic formis stable. The stabilisation is typically carried out by adding yttriumoxide (Y₂O₃), but also MgO, CaO, CeO₂, In₂O₃, Gd₂O₃, and Al₂O₃ have beenused. Previously, YSZ thin film layers have been produced, for example,by metal-organic chemical vapour deposition (MOCVD) (Garcia, G. et al.,Preparation of YSZ layers by MOCVD: Influence of experimental parameterson the morphology of the film, J. Crystal Growth 156 (1995), 426) ande-beam evaporation techniques (cf. Mattheé, Th. et al., Orientationrelationships of epitaxial oxide buffer layers on silicon (100) forhigh-temperature superconducting YBa₂Cu₃O_(7-x) films, Appl. Phys. Lett.61 (1992), 1240).

Atomic layer deposition (ALD) can be used for producing binary oxidethin films. ALD, which originally was known as atomic layer epitaxy(ALE) is a variant of traditional CVD. The method name was recentlychanged from ALE into ALD to avoid possible confusion when discussingabout polycrystalline and amorphous thin films. Equipment for ALD issupplied under the name ALCVD™ by ASM Microchemistry Oy, Espoo, Finland.The ALD method is based on sequential self-saturating surface reactions.The method is described in detail in U.S. Pat. Nos. 4,058,430 and5,711,811. The growth benefits from the usage of inert carrier andpurging gases which makes the system faster.

When ALD type process is used for producing more complicated compounds,all components may not have, at the same reaction temperature range, anALD process window, in which the growth is controlled. Mölsä et al. havediscovered that an ALD type growth can be obtained when growing binarycompounds even if a real ALD window has not been found, but the growthrate of the thin film depends on the temperature (Mölsä, H. et al., Adv.Mat. Opt. El. 4 (1994), 389). The use of such a source material andreaction temperature for the production of solid solutions and dopedthin films may be found difficult when a precise concentration controlis desired. Also the scaling of the process becomes more difficult, ifsmall temperature changes have an effect on the growth process.

Mölsä et al. (Mölsä, H. et al., Adv. Mat. Opt. El. 4 (1994), 389)disclosed a process for growing Y₂O₃ by ALE-method. They used Y(thd)₃(thd=2,2,6,6-tetramethyl-3,5-heptanedione) as the yttrium sourcematerial and ozone-oxygen mixture as the oxygen source material in atemperature range of 400-500° C. As already discussed, no ALE windowcould be found since the growth rate increased steadily from 0.3 Å/cycleto 1.8 Å/cycle with increasing temperature.

Ritala et al. (Ritala, M. and Leskelä, M., Appl. Surf Sci. 75 (1994),333) have disclosed a process for growing ZrO₂ by an ALD type process.ZrCl₄ was used as the zirconium source material and water was used asthe oxygen source material. The temperature in the process was 500° C.and the growth rate was 0.53 Å/cycle.

SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate the problems ofprior art and to provide a novel process for producingyttrium-stabilised zirconium oxide (YSZ) thin films.

This and other objects together with the advantages thereof are achievedby the present invention as hereinafter described and claimed.

The present invention is based on the finding that yttrium oxide andzirconium oxide can be grown by an ALD type method so that the filmgrowth is in accordance with the principles of ALD so as to form anyttrium-stabilised zirconium oxide thin film.

More specifically, the process for producing YSZ thin films ischaracterised by what is stated in the characterising part of claim 1.

A number of considerable advantages are achieved by means of the presentinvention.

The growth rate of the yttrium-stabilised zirconium oxide thin film ishigh, e.g., the growth rate of ALD thin film was approximately 25%higher than would be expected based on the growth rates of ZrO₂ andY₂O₃.

The temperatures used in the present invention are low compared with theprocesses of prior art, which reduces the cost of the productionprocess.

A film grown with the present process exhibits good thin filmproperties. Thus, the oxide films obtained have an excellentconformality even on uneven surfaces. The method also provides anexcellent and automatic self-control for the film growth.

The ALD grown yttrium-stabilised zirconium oxide thin films can be used,for example, as insulators in electronics and optics. For example, infield emission displays (FED) it is preferred that insulating oxideswhich have a smooth surface, are used. It is also possible to use theYSZ thin films as solid electrolytes in gas sensors and fuel cells.Particularly suitably the YSZ thin films are used as gate oxides inmicroelectronic devices, and as capacitor in dynamic random accessmemory (DRAM).

Next, the invention is described in detail with the aid of the followingdetailed description and by reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents the growth rate of Y₂O₃ as a function of the growthtemperature.

FIG. 2 presents the growth rate of Y₂O₃ as a function of the pulse timesof the source materials.

FIG. 3 presents the thickness of a Y₂O₃ thin film in nm as a function ofthe number of reaction cycles.

FIG. 4 presents the growth rate of ZrO₂ as a function of growthtemperature.

FIG. 5 presents the growth rate of ZrO₂ as a function of pulse times.

FIG. 6 presents the thickness of the ZrO₂ film as a function of thenumber of reaction cycles.

FIG. 7 presents the X-ray diffraction (XRD) patterns of ZrO₂ thin filmsgrown at 300° C. and 450° C.

FIG. 8 presents the pulsing sequences of ZrO₂, YSZ and Y₂O₃ thin films.

FIG. 9 presents the growth rate of a YSZ thin film as a function of Y₂O₃content in the film.

FIG. 10 presents the XRD pattern of a YSZ thin film (thickness 90 nm)grown on a (100) silicon substrate.

FIG. 11 presents the change of the d-value (interplanar spacing) of the(200) plane in the XRD pattern of a YSZ film as a function of Y₂O₃concentration.

FIG. 12 presents the chloride concentration in a YSZ thin film as afunction of the concentration of Y₂O₃.

FIG. 13 presents IR-spectra of (100) silicon substrate (a), YSZ thinfilm (10 wt-% of Y₂O₃, thickness 120 nm, b) and a subtracted spectrum(c).

FIG. 14 presents the dependency of the wavenumber in the mid-IR-area onthe concentration of Y₂O₃.

FIG. 15 presents the Y/Zr ratio measured with different analysismethods.

DETAILED DESCRIPTION OF THE INVENTION Definitions

For the purposes of the present invention, an “ALD-type process”designates a process in which growth of material from gaseous orvaporized source chemicals onto a surface is based on sequential andalternating self-saturating surface reactions. The principles of ALD aredisclosed, e.g., in U.S. Pat. Nos. 4,058,430 and 5,711,811.

“Reaction space” is used to designate a reactor or reaction chamber inwhich the conditions can be adjusted so that growth by ALD is possible.

“ALD window” is used to designate the temperature range in which thegrowth of a thin film takes place according to the principles of ALD.One indication of thin film growing according to the ALD principles isthe fact that the growth rate remains essentially constant over thetemperature range.

“Thin film” is used to designate a film which is grown from elements orcompounds that are transported as separate ions, atoms or molecules viavacuum, gaseous phase or liquid phase from the source to the substrate.The thickness of the film depends on the application and it varies in awide range, e.g., from one molecular layer to 800 nm or up to 1 μm oreven over that.

The growth process

According to the present invention, the oxide thin films are produced byan ALD method. Thus, a substrate placed in a reaction chamber issubjected to sequential, alternately repeated surface reactions of atleast two vapor-phase reagents for the purpose of growing a thin filmthereon.

The conditions in the reaction space are adjusted so that no gas-phasereactions, i.e., reactions between gaseous reagents, occur, only surfacereactions, i.e., reactions between species adsorbed on the surface ofthe substrate and a gaseous reagent. Thus, the molecules of oxygensource material react with the adsorbed metal source compound layer onthe surface. This kind of growth is in accordance with the principles ofALD.

According to the present process the vapour-phase pulses of the metalsource material and the oxygen source material are alternately andsequentially fed to the reaction space and contacted with the surface ofthe substrate fitted into the reaction space. The “surface” of thesubstrate comprises initially the surface of the actual substratematerial which optionally has been pre-treated in advance, e.g., bycontacting it with a chemical for modifying the surface propertiesthereof. During the growth process of the thin films, the previous metaloxide layer forms the surface for the following metal oxide layer. Thereagents are preferably fed into the reactor with the aid of an inertcarrier gas, such as nitrogen.

Preferably, and to make the process faster, the metal source materialpulse and the oxygen source material pulse are separated from each otherby an inert gas pulse, also referred to as gas purge in order to purgethe reaction space from the unreacted residues of the previous chemicaland the reaction products. The inert gas purge typically comprises aninactive gas, such as nitrogen, or a noble gas, such as argon.

Thus, one pulsing sequence (also referred to as a “cycle” or “reactioncycle”) preferably consists essentially of

-   -   feeding a vapour-phase pulse of a metal source chemical with the        help of an inert carrier gas into the reaction space;    -   purging the reaction space with an inert gas;    -   feeding a vapour-phase pulse of an oxygen source material into        the reaction space; and    -   purging the reaction space with an inert gas.

The purging time is selected to be long enough to prevent gas phasereactions and to prevent metal oxide thin film growth rates higher thanoptimum ALD growth rate per cycle for said oxide.

The deposition can be carried out at normal pressure, but it ispreferred to operate the method at reduced pressure. The pressure in thereactor is typically 0.01-20 mbar, preferably 0.1-5 mbar.

The substrate temperature has to be low enough to keep the bonds betweenthin film atoms intact and to prevent thermal decomposition of thegaseous or vaporised reagents. On the other hand, the substratetemperature has to be high enough to keep the source materials in gasphase, i.e., condensation of the gaseous or vaporised reagents must beavoided. Further, the temperature must be sufficiently high to providethe activation energy for the surface reaction. When growing zirconiumoxide on a substrate, the temperature of the substrate is typically250-500° C., preferably 275-450° C., and in particular 275-325° C. Thetemperature range used for growing Y₂O₃ on a substrate is typically200-400° C., preferably 250-350° C. The YSZ films are typically grown at250-400° C., preferably at 275-350° C., and in particular at 275-325° C.

At these conditions, the amount of reagents bound to the surface will bedetermined by the surface. This phenomenon is called “self-saturation”.

Maximum coverage on the substrate surface is obtained when a singlelayer of metal source chemical molecules is adsorbed. The pulsingsequence is repeated until an oxide film of predetermined thickness isgrown.

The source temperature is preferably set below the substratetemperature. This is based on the fact that if the partial pressure ofthe source chemical vapour exceeds the condensation limit at thesubstrate temperature, controlled layer-by-layer growth of the film islost.

The amount of time available for the self-saturating reactions islimited mostly by the economical factors such as required throughput ofthe product from the reactor. Very thin films are made by relatively fewpulsing cycles and in some cases this enables an increase of the sourcechemical pulse times and, thus, utilization of the source chemicals witha lower vapour pressure than normally.

The substrate can be of various types, for example sheet-formed orpowder-like. Examples include silicon, silica, coated silicon, coppermetal, and various nitrides, such as metal nitrides.

The YSZ thin films grown according to the process of the presentinvention are typically (100) oriented.

Chlorine residues can be found in thin films comprising zirconium and/oryttrium, when one or more of the source materials contains chlorine. Inthe YSZ thin films produced according to the present invention theconcentration of Cl in the films is typically 0.05-0.25 wt-%. It wassurprisingly found out in the connection of the present invention, thatwhen the concentration of yttrium in the formed film was low, i.e.,below 20 wt-%, and in particular below 15 wt-%, the chloride content ofthe formed thin film was lower than that of a film consistingessentially of ZrO₂.

In the pulsing sequence described above, the metal source chemical iseither a zirconium source material or an yttrium source material. Thus,in the growth process of the present invention, yttrium oxide andzirconium oxide are grown on a substrate.

According to a preferred embodiment of the present invention, anyttrium-stabilised zirconium oxide thin film is formed. Thus, during thegrowth of the thin film, at least one pulsing cycle described above willbe carried out using an yttrium source chemical as the metal sourcechemical, and at least one pulsing cycle described above will be carriedout using a zirconium source chemical as the metal source chemical.

The pulsing ratio between yttrium source chemical and zirconium sourcechemical can be selected so as to obtain the desired properties to thethin film. Typically, the pulsing ratio Y:Zr is from 1:10 to 10:1,preferably from 1:5 to 5:1, more preferably from 1:3 to 3:1, and mostpreferably the pulsing ratio is approximately 1:1.

FIG. 8 presents pulsing sequences which can be used for growing ZrO₂,YSZ and Y₂O₃ thin films. In FIG. 8(b) the pulsing sequence for pulsingratio Y:Zr=1:2 for the YSZ film is depicted.

FIG. 9 presents the growth rate of a YSZ thin film compared to a valuecalculated for separate oxides as a function of weight percentage ofY₂O₃ in the film. The value to which the growth rate of YSZ is comparedis calculated by adding together the growth rates of Y₂O₃ and ZrO₂ ateach pulsing ratio of Y₂O₃: ZrO₂, and this calculated value represents100% in the figure. Thus, the figure shows which effect the Y₂O₃:ZrO₂pulsing ratio has on the growth rate and yttrium concentration of theYSZ thin film.

According to one embodiment of the invention, when growing a YSZ thinfilm the first pulsing cycle on a substrate is carried out using anyttrium source chemical as the metal source chemical.

According to another embodiment of the invention, when growing a YSZthin film the first pulsing cycle on a substrate is carried out using azirconium source material as the metal source chemical.

The Source Materials

Gaseous or volatile compounds of yttrium and zirconium are used as metalsource materials in the process of the present invention.

Since the properties of each metal compound vary, the suitability ofeach metal compound for the use in the process of the present inventionhas to be considered. The properties of the compounds are found, e.g.,in N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, 2^(nd)edition, Pergamon Press, 1997.

The metal source material has to be chosen so that requirements forsufficient vapour pressure, the sufficient thermal stability atsubstrate temperature and sufficient reactivity of the compounds arefulfilled.

Sufficient vapour pressure means that there must be enough sourcechemical molecules in the gas phase near the substrate surface to enablefast enough self-saturating reactions at the surface.

In practice sufficient thermal stability means that the source chemicalitself must not form growth-disturbing condensable phases on thesubstrates or leave harmful levels of impurities on the substratesurface through thermal decomposition. Thus, one aim is to avoidnon-controlled condensation of molecules on substrates.

Further selecting criteria may include the availability of the chemicalin a high purity, and the easiness of handling, inter al., reasonableprecautions.

In addition, the quality of the by-products resulting from the ligandexchange reaction needs to be considered. It is important that thereaction product is essentially gaseous. By this it is meant that theby-products possibly formed in the ligand exchange reaction are gaseousenough to be moved from the reaction space with the aid of the inertpurging gas, which means that they will not remain as impurities in thefilms.

1. Yttrium Source Material

The yttrium source material is typically selected from the group ofmaterials having general formula (I) or (II):YX₃  (I)YX₃B  (II)wherein

-   -   X is selected from the group of following:    -   i) diketone coordinated from oxygen (i.e., β-diketonate) of        formula (III)    -   wherein        -   R′ and R″ are typically the same and are selected for            example from the group of linear or branched C₁-C₁₀ alkyl            groups, in particular linear or branched C₁-C₆ alkyl groups,            and most preferably from the group of —CH₃, —C(CH₃)₃, —CF₃            and —C(CF₃)₃,    -   ii) cyclopentadienyl,    -   iii) derivative of cyclopentadienyl according to formula (IV):        C₅H_(5-y)R′″_(y)  (IV)    -   wherein    -   R′″ is selected for example from the group of linear or branched        C₁-C₁₀ alkyl groups, preferably C₁-C₆ alkyl groups, in        particular methyl (—CH₃), ethyl, propyl, butyl, pentyl, and an        alkyl having a longer carbon chain, alkoxy, aryl, amino, cyano        and silyl group, and    -   y is an integer 1-5, and    -   B is a neutral adduct ligand, which binds to the center atom        from one or more atoms. Typically, B is hydrocarbon,        oxygen-containing hydrocarbon (such as ether),        nitrogen-containing hydrocarbon (such as bipyridine,        phenantroline, amine or polyamine)

According to one embodiment of the present invention,Y(cot)Cp*(cot=cyclo-octatetraenyl and Cp*=pentamethyl cyclopentadienyl)is used as the yttrium source material.

According to a preferred embodiment of the present invention, Y(thd)₃(thd=2,2,6,6-tetramethyl-3,5-heptanedione) is used as the yttrium sourcematerial.

2. Zirconium Source Material

The zirconium source material is typically selected from the group ofzirconium halides and zirconium compounds comprising at least one carbonatom.

The zirconium source material is typically selected from the grouphaving the general formula (V)R₂ZrX₂  (V)wherein

-   -   R is selected from the group of cyclopentadienyl (C₅H₅) and its        derivatives having the formula (IV).    -   The ligands R are optionally bridged (-Cp-A-Cp-), wherein A is        methyl, an alkyl group of formula (CH₂)_(n), n=2-6, preferably 2        or 3) or a substituted hydrocarbon such as C(CH₃)₂.    -   X is selected from the group of following ligands:    -   i) halides (F, Cl, Br, I),    -   ii) hydrogen (—H), linear or branched C₁-C₁₀ alkyl groups,        preferably C₁-C₆ alkyl groups, in particular methyl (—CH₃),        ethyl, propyl, butyl or a longer hydrocarbon chain,    -   iii) methoxy (—OCH₃) or other linear (e.g. —OC₃H₇) or branched        alkoxides,    -   iv) amines (—NR₂), and    -   v) acetates (—OCOR, e.g. —OCOCF₃).    -   According to one embodiment of the invention, X-ligands are        combinations of the compounds identified above. Thus, the        zirconium source material is optionally Cp₂Zr(OR′″)_(x)Cl_(2-x)        or Cp₂ZrClH).

The following preferred combinations of X and R can also be used in thepresent invention:

-   -   X═R═Cl or Br, i.e, compound is a tetrahalide,    -   X═R═OR″, i.e., the compound is a zirconium alkoxide,    -   X═R═Cp, i.e., the compound is tetracyclopentadienezirconium,        and/or    -   X═R=diketonate, coordinated from oxygen, having a formula (III).

Preferably, the zirconium source material used in the present inventionis zirconium tetrachloride (ZrCl₄) or dicyclopentadienyl zirconiumdichloride (Cp₂ZrCl₂).

3. Oxygen Source Material

The oxygen source material may be any oxygen compound usable in the ALEtechnique. Preferable oxygen source materials include water, oxygen andhydrogen peroxide, and aqueous solutions of hydrogen peroxide. Ozone(O₃) is an especially preferable oxygen source material, also as mixturewith oxygen (O₂). It is known on the basis of the literature that, ifozone is used as the precursor for oxygen, a denser layer of material isobtained from the forming oxides, and thereby the permittivity of theoxide thin film can be improved.

One or more of the following compounds may also be used as the precursorfor oxygen:

-   -   oxides of nitrogen, such as N₂O, NO, and NO₂,    -   halide-oxygen compounds, for example chlorine dioxide (ClO₂) and        perchloric acid (HClO₄),    -   peracids (—O—O—H), for example perbenzoic acid (C₆H₅COOOH) and        peracetic acid (CH₃COOOH),    -   alkoxides,    -   alcohols, such as methanol (CH₃OH) and ethanol (CH₃CH₂OH), and    -   various radicals, for example oxygen radical (O⁻) and hydroxyl        radical (−OH).

According to a preferred embodiment of the present invention, a YSZ thinfilm is grown by an ALD type method using Y(thd)₃ as the yttrium sourcematerial, dicyclopentadienyl zirconium dichloride (Cp₂ZrCl₂) as thezirconium source material and ozone or a mixture of O₃ and O₂ as theoxygen source material.

According to another preferred embodiment, a YSZ thin film is grown byan ALD type method using Y(thd)₃ as the yttrium source material and amixture of O₃ and O₂ as the oxygen source material, and zirconiumtetrachloride (ZrCl₄) as the zirconium source material and water as theoxygen source material.

The following examples illustrate the invention further.

EXAMPLES Experimental Conditions and Analysis Equipment

In the examples, Y(thd)₃ and dicyclopentadienyl zirconium dichloride(Cp₂ZrCl₂) (Strem Chemicals) were used as the metal source materials.Y(thd)₃ was prepared according to the teaching of Eisentraut and Sievers(Eisentraut, K. J. and Sievers, R. E., J. Am. Chem. Soc. 87 (1965),5254). The source materials were analysed thermogravimetrically (TG/DTA,Seiko SSC 5200) at a pressure of 1 mbar.

The thin films were grown in MC-120 and F-120 reactors (ASMMicrochemistry Oy, Espoo, Finland) and N₂ (5.0, Aga) was used as thecarrier gas. Ozone, produced with an ozone generator (Fisher 502) fromO₂ (5.0, Aga), was used as the oxidiser. (100) oriented silicon (OkmeticOy, Finland) and lime glass were used as substrates. The growing ofseparate yttrium oxides and zirconium oxides was examined as thefunction of temperature and the suitability of the source materials wasconfirmed by experimenting with pulsing times in the range of 0.5-4seconds.

The crystallinity and orientation of the grown Y₂O₃, ZrO₂ and YSZ thinfilms were analysed by X-ray diffraction (XRD, Philips MPD1880, CuK_(α)). The Y and Zr contents and the possible impurities weredetermined by X-ray fluoresence (XRF, Philips PW1480) using UniQuant 4.0software and by Scanning Electron Microscopy with Energy DispersiveX-ray analysis (SEM-EDX) using STRATA software. YSZ thin films were alsoanalysed by X-ray photon spectroscopy (XPS, AXIS 165, Kratos Analytical)using monochromated Al K_(α) radiation. Both wide scan spectra and HiRes(high resolution) spectra from areas C 1s, O 1s, Zr 3d and Y 3d weredetermined. The area of the measured sample was approximately 1 mm², andmeasurements were carried out from several points.

The thicknesses of the thin films were determined either with HitachiU-2000 UV-Vis spectrophotometer and with optical fitting method astaught by Ylilammi, M. and Ranta-Aho, T. in Thin Solid Films 232 (1993),56 or by profilometry (the Y₂O₃ thin films) (Sloan Dektak SL3030, VeecoInstruments) by etching with a solution of HCl the appropriate stepsusing a photoresist (AZ 1350H, Hoechst) as a mask.

The thin films were analysed also by Nicolet Magna-IR 750FT-IR-spectrophotometer using a DTGS detector and a DRIFTS accessory(Spectra Tech Inc.). From the samples prepared on a approximately0.5×0.5 cm² (100) silicon substrate mid-IR-area spectra were measuredwith 2 cm⁻¹ resolution and signal-averaging of 64 scans were used. Thebackground was measured with the diffuse alignment mirror of the device(SpectraTech no: 7004-015). The spectra of the silicon wafer with anative oxide was subtracted from the spectra of the samples. Theinterference in the spectra resulting from water and CO₂ residues in theIR apparatus was eliminated by purging with dry air. Smoothing of themeasured spectra was carried out when necessary.

Example 1 The Preparation and Analysis of Yttrium Oxide (Y₂O₃) ThinFilms

Y₂O₃ thin films were grown by ALD method at a temperature of 250-350° C.The growth rate of the Y₂O₃ thin films was 0.23 Å/cycle.

For Y₂O₃ thin film growth from Y(thd)₃ an ALD window was found in whichthe growth rate remained essentially constant in the temperature rangeof 250-350° C. The source material temperature was 120° C., the pulsingtimes were 0.8 s for Y(thd)₃ and 3.0 s for O₃, and the purging aftereach source material pulse lasted 1.0 s. This is also presented in FIG.1, wherein the growth rate of Y₂O₃ in Å per cycle is depicted as afunction of growth temperature.

FIG. 2 depicts the growth rate of Y₂O₃ as Å per cycle as a function ofthe pulse times of the source materials. The figure shows how the growthrate remains essentially constant when the pulsing time of Y(thd)₃ isapproximately 0.5 s (during this experiment, the O₃ pulse as maintainedat 1.5 s) or more and the pulsing time of O₃ is approximately 1.0 s ormore (during this experiment, the Y(thd)₃ pulse was maintained at 0.8s). The temperature of the yttrium source material was approximately120° C., and the growth temperature was 300° C. The purging after eachsource material pulse varied from 0.8 to 2.0 s, increasing withincreased pulse time.

In FIG. 3 the thickness of a Y₂O₃ thin film in nm is presented as afunction of the number of reaction cycles. The film was deposited at300° C., and the temperature of the source material Y(thd)₃ was 120° C.The pulse times were 0.8 s for Y(thd)₃ and 3.0 s for O₃. The purgingafter each source material pulse lasted 1.0 s. It can be seen from FIG.3 that the thickness of the film is linearly dependent on the number ofgrowth cycles.

The Y₂O₃ films grown in the ALD window of 250-350° C. were (100)oriented. In the films grown at higher temperatures also (111) and (440)orientations were detected. The growth at temperatures higher than 400°C. yielded results similar to those obtained in prior art (Mölsä, H. etal., Adv. Mat. Opt. El. 4 (1994), 389). The orientation or crystallinityof the thin films did not vary according to the pulsing times of thesource materials.

Example 2 The Preparation and Analysis of Zirconium Oxide (ZrO₂) ThinFilms

Zirconium oxide thin films were produced using Cp₂ZrCl₂ as the zirconiumsource material. The temperature of the source material was 140° C. TheZrO₂ thin films could be grown according to the principles of ALD attemperatures of 275-325° C. and at 400-450° C. In the first range, agrowth rate of 0.48 Å/cycle was obtained, and in the second range, thegrowth rate was 0.53 Å/cycle.

This can also be seen in FIG. 4 which presents the growth rate of a ZrO₂thin film as a function of growth temperature. In this experiment, thetemperature of the source material Cp₂ZrCl₂ was 140-150° C. The pulsingtimes of Cp₂ZrCl₂ and O₃ were 0.8 s and 3.0 s, respectively. The purgingafter each source material pulse lasted 1.0 s.

The pulsing times of the source materials were changed in someexperiments. A Cp₂ZrCl₂ pulse of 1.0 s saturated the surface of thesubstrate. An O₃ pulse of 1.5 s was needed to complete the reactioncycle. FIG. 5 depicts the growth rate of ZrO₂ in A per cycle as afunction of the pulse time. The growth temperature was 300° C. and thetemperature of source material Cp₂ZrCl₂ was 140-150° C. The purging timewas 1.0 s. The figure shows how the growth rate remains essentiallyconstant when the pulsing time of Cp₂ZrCl₂ is approximately 0.7 s ormore (during these experiments, the pulsing time of O₃ was 3.0 s) andthe pulsing time of O₃ is approximately 1.5 s or more (during theseexperiments, the pulsing time of CpZrCl₂ was 0.8 s).

In FIG. 6 the thickness of a ZrO₂ thin film in nm is presented as afunction of the number of reaction cycles. The film was deposited at300° C., the temperature of the source material Cp₂ZrCl₂ was 140-150° C.The pulse times were 0.8 s for Cp₂ZrCl₂ and 3.0 s for O₃. The purgingafter each source material pulse lasted 1.0 s. It can be seen from FIG.6 that the thickness of the film is linearly dependent on the number ofgrowth cycles.

XRF was used to analyse possible Cl residues present in the ZrO₂ thinfilms. In the thin films grown on a silicon or glass substrate at250-275° C., approximately 0.1 wt-% of Cl was present. The thin filmsgrown at 300-325° C. exhibited a chlorine content of approximately0.06-0.07 wt-%. For films grown at temperatures higher than 325° C.chloride was not detected, i.e., the chlorine content was under thedetection limit, i.e., approximately 0.02 wt-% or less.

XRD was used to analyse the ZrO₂ films grown at different temperatures.The ZrO₂ thin films grown on silicon or glass substrate at temperaturesbelow 300° C. were almost amorphous. Only very weak peaks, which couldbe interpreted as reflections of monoclinic ZrO₂ were shown in the filmgrown at 275° C. In the film grown at 300° C. the peaks could beidentified as reflections of monoclinic or cubic ZrO₂ phase. When thegrowth temperature was up to 450° C., the monoclinic (−111) reflectionwas even stronger. The XRD patterns for the films grown at 300° C. and450° C. on a silicon substrate are presented in FIG. 7. The pattern forthe film grown at 300° C. is the one below. The thicknesses of filmsgrown at 300° C. and 450° C. are 120 and 90 nm, respectively. Theabbreviations used in the identification of the phases are as follows:M=monoclinic, C=cubic. The identification was according to JCPDS cards36-420 and 27-997 (Joint Committee on Powder Diffraction Standards(JCPDS), 1990).

Example 3 The Preparation and Analysis of Yttrium-Stabilised ZirconiumOxide Thin Films

YSZ thin films were grown at a temperature of 300° C. with differentpulsing programmes. In each pulsing programme the number of pulsingsequences consisting of Y(thd)₃-pulse/purge/O₃-pulse/purge was variedwith relation to the number of pulsing sequences consisting ofCp₂ZrCl₂-pulse/purge/O₃-pulse/purge.

The quality or the growth rate of the thin film did not depend on thechoice of the metal source material first deposited on the surface ofthe substrate.

The growth rate of the yttrium-stabilised zirconium oxide was dependenton the Y/Zr pulsing ratio. If the growth rate of YSZ is compared withthe summed growth rates of separate oxides, it is noticed that the at apulsing ratio of 1:1, the growth rate is approximately 25% greater thanthe value calculated from the growth rates of separate oxides. When thenumber of yttrium pulsing sequences is increased, i.e., when the yttriumcontent in the thin film increases, the growth rate approaches thecalculated value. This can also be concluded from FIG. 9.

The YSZ films grown at 300° C. were cubic, mainly (100) oriented, but,as FIG. 10 shows, also (111), (220) and (311) reflections were detected.FIG. 10 discloses an XRD pattern for a YSZ thin film of a thickness 90nm. The film was grown on a (100) silicon substrate at 300° C. The Y/Zrpulsing ratio was 1:1. The phase is identified according to JCPDS-card30-1468. The position of peaks in the XRD pattern change as a functionof the concentration of yttrium, since the size of the unit cellchanges. The JCPDS reference value (card 30-1468) for the (200)reflection of Y_(0.15)Zr_(0.85)O_(1.93) is d=2.571 Å. FIG. 11 shows howthe (200) peak in the XRD pattern of a YSZ thin film changes as theY₂O₃/ZrO₂ ratio is changed. The dashed line in FIG. 11 is a referenceline drawn via the d-values of pure oxides obtained from literature.

The chlorine content of the YSZ films grown at 300° C. was analysed withXRF. At low concentrations of Y the amount of Cl in the films seemed tobe slightly lower than in the films consisting essentially of ZrO₂. Inthe range 20-50 wt-% of Y₂O₃ in the thin film the increase in the amountof yttrium in the film resulted in an increase of the amount of Clincorporated in the film. This can also be seen in FIG. 12. The highestconcentration of chlorine (0.23 wt-%) in the YSZ thin film was detectedwhen the yttrium oxide concentration was 50 wt-%.

The IR-spectra measured for the YSZ films in the mid-IR-area mostlyshowed only the peaks resulting from the silicon substrate at differentwave numbers. The actual peaks resulting from the YSZ film could bedetected by subtracting the IR-spectra of the Si-substrate (cf. FIG.13). In the subtracting, the peak due to Si—O bond at 1100 cm⁻¹ did notcompletely disappear. FIG. 14 shows how a distinct shift to higher wavenumbers can be detected in the analysed films as the concentration ofyttrium decreases. The reference value of Y₂O₃ absorption is 613 cm⁻¹.

A series of grown Y₂O₃, ZrO₂ and YSZ films was analysed with X-rayphotoelectron spectroscopy (XPS). The contents of Y₂O₃ in the sampleswere 0, 3, 10, 30 or 100 wt-%. Small amounts of carbon and oxygen (CO₂)were detected on the surface. This is typical for samples handled inair. The spectra measured from the surface was used to calculate theatom compositions on the surface, and the atom ratio Y/Zr which wascompared with the results obtained from X-ray fluorescence (XRF)measurements. This comparison is presented in FIG. 15, wherein theY₂O₃/ZrO₂ ratio calculated from XRF results is in the x-axis and theY/Zr ratio according to the XPS results is in the y-axis. The line isdrawn based on the XPS-HiRes measurements.

1. An atomic layer deposition method for forming a film on a substrate,comprising: providing the substrate in a reaction space; depositing azirconium oxide film on the substrate by exposing the substrate in thereaction space to temporally separated pulses of a zirconium-containingmaterial and an oxygen-containing material, wherein the zirconium sourcematerial comprises a ligand selected from the group consisting of acyclopentadienyl and a cyclopentadienyl derivative; and maintaining thesubstrate at a temperature below a decomposition temperature of thezirconium-containing material during depositing the zirconium oxidefilm.
 2. The method of claim 1, wherein the zirconium-containingmaterial comprises two or more ligands selected from the groupconsisting of a cyclopentadienyl and a cyclopentadienyl derivative. 3.The method of claim 2, wherein the zirconium-containing materialcomprises zirconium with R and X groups as ligands, wherein R isselected from the group consisting of a cyclopentadienyl and acyclopentadienyl derivative; and X is selected from the group consistingof F, Cl, Br, I, H, a linear alkyl group, a branched alkyl group, alinear alkoxide, a branched alkoxide, an amine and an acetate.
 4. Themethod of claim 3, wherein the linear or branched alkyl groups comprisea 1-10 carbon hydrocarbon chain.
 5. The method of claim 3, wherein thelinear alkoxide is a methoxy group.
 6. The method of claim 3, whereinthe linear alkyl group is a methyl group.
 7. The method of claim 3,wherein the zirconium-containing material comprises two different Xgroups.
 8. The method of claim 7, wherein a first X group is a methylgroup and a second X group is a methoxy group.
 9. The method of claim 3,wherein the ligands R are bridged together.
 10. The method of claim 9,wherein the ligands R are bridged together with a bridge selected fromthe group consisting of a methyl group, an alkyl group, and asubstituted hydrocarbon.
 11. The method of claim 3, wherein thecyclopentadienyl derivative has the general formula C₅H_(5-y)R′″_(y),wherein R′″ is selected from the group consisting of a linear alkylgroup, a branched alkyl group, an alkoxy group, an aryl group, an aminogroup, a cyano group, and a silyl group; and y is an integer between 1and
 5. 12. The method of claim 11, wherein the linear or branched alkylgroups comprise a 1-10 carbon hydrocarbon chain.
 13. The method of claim11, wherein the linear alkyl group is a methyl group.
 14. The method ofclaim 3, wherein the cyclopentadienyl derivative comprises a ligand R′″,wherein R′″ is selected from the group consisting of a linear alkyl or abranched alkyl group.
 15. The method of claim 14, wherein the linearalkyl group is a methyl group.
 16. The method of claim 1, wherein theoxygen-containing material comprises O₃.
 17. The method of claim 1,wherein the oxygen-containing material comprises H₂O.
 18. The method ofclaim 1, wherein the oxygen-containing material is one or moreprecursors selected from the group consisting of an oxide of nitrogen, ahalide-oxygen compound, a peracid, an alkoxide, an alcohol and anoxygen-containing radical.
 19. The method of claim 18, wherein theoxygen-containing radical is an oxygen radical or a hydroxyl radical.20. The method of claim 1, wherein depositing the zirconium oxide filmis performed at a deposition temperature in a range between 250 and 500°C.
 21. The method of claim 20, wherein depositing the zirconium oxidefilm is performed at a deposition temperature in a range between 275° C.and 325° C.
 22. The method of claim 1, further comprising exposing thesubstrate to pulses of a yttrium-containing material to form ayttrium-stabilized zirconium oxide film.
 23. The method of the claim 22,wherein exposing the substrate in the reaction space to temporallyseparated pulses of the zirconium-containing material, theoxygen-containing material and the yttrium-containing materialconstitute a cycle, further comprising performing a plurality of cylces.24. An atomic layer deposition method, comprising: forming a zirconiumoxide film on a substrate by alternatingly introducing pulses of azirconium cyclopentadienyl compound and an oxygen-containing materialinto a reaction space containing a substrate, wherein the substrate isat a temperature above a condensation temperature of the zirconiumcyclopentadienyl compound and below a decomposition temperature of thezirconium cyclopentadienyl compound.
 25. The method of claim 24, whereinthe zirconium cyclopentadienyl compound comprises a cyclopentadienylderivative.
 26. The method of claim 24, further comprising purging thereaction space between the pulses of the zirconium cyclopentadienylcompound and the oxygen-containing material.
 27. The method of claim 24,wherein introducing the zirconium cyclopentadienyl compound depositszirconium on the substrate, wherein introducing the oxygen-containingmaterial oxidizes the zirconium deposited on the substrate.
 28. Themethod of claim 24, further comprising exposing the substrate to pulsesof a yttrium-containing material to form a three element thin film. 29.A method for producing an integrated circuit comprising: forming ayttrium stabilized zirconium oxide thin film over a substrate comprisinga partially fabricated integrated circuit by an atomic layer deposition(ALD)-type process comprising alternately feeding into said reactionspace vapor phase pulses of a first yttrium source material, a secondzirconium source material, and at least one oxygen source materialcapable of forming an oxide with the first yttrium source material andthe second zirconium source material.
 30. The method of claim 29,wherein the thin film is a capacitor dielectric.
 31. The method of claim29, wherein the thin film is a gate oxide layer.
 32. A method of forminga capacitor for a random access memory (RAM) device by an atomic layerdeposition (ALD)-type process, said method comprising: providing areaction chamber in an ALD reactor with a substrate comprising apartially fabricated capacitor; and forming a yttrium stabilizedzirconium oxide thin film over the substrate by a process comprising:alternatively feeding into said reaction space vapor phase pulses of afirst metal source material, a second metal source material and at leastone oxygen source material capable of forming an oxide with the firstmetal source material and the second metal source material, wherein saidfirst metal source material is a yttrium source material and said secondmetal source material is a zirconium source material.
 33. The method ofclaim 32, wherein the yttrium source material is Y(thd)₃, and thezirconium source material is dicyclopentadienyl zirconium dichloride(Cp₂ZrCl₂), and the oxygen source material is selected from the groupconsisting of O₃ and a mixture of O₂ and O₃.
 34. The method of claim 32,wherein the random access memory device is a dynamic random accessmemory (DRAM) device.
 35. A method of forming a gate oxide for atransistor comprising: providing a reaction chamber of an atomic layerdeposition (ALD) reactor with a substrate comprising a partiallyfabricated integrated circuit; and forming a yttrium stabilizedzirconium oxide thin film over the substrate by an ALD-type processcomprising: alternately feeding into the reaction space an yttriumsource material, a zirconium source material and an oxygen sourcematerial, wherein the oxygen source material is capable of forming anoxide with the yttrium source material and the zirconium sourcematerial.
 36. An atomic layer deposition method for forming a film on asubstrate, comprising: providing the substrate in a reaction space; anddepositing a zirconium oxide film on the substrate by exposing thesubstrate in the reaction space to temporally-separated pulses of azirconium-containing precursor and an oxygen-containing precursor,wherein the zirconium-containing precursor comprises a plurality ofligands, wherein at least one of ligands is an amine.
 37. The method ofclaim 36, wherein the zirconium-containing precursor comprises aplurality of amine ligands.
 38. The method of claim 37, wherein theplurality of ligands each comprises a hydrocarbon group selected fromthe group consisting of a methyl group and an alkyl group.
 39. Themethod of claim 38, wherein the alkyl group is a linear alkyl group. 40.The method of claim 38, wherein the zirconium-containing precursorcomprises a ligand selected from the group consisting of acyclopentadienyl and a cyclopentadienyl derivative.
 41. The method ofclaim 40, wherein the cyclopentadienyl derivative has the generalformula C₅H_(5-y)R′″_(y), wherein wherein R′″ is the hydrocarbon; and yis an integer between 1 and
 5. 42. The method of claim 36, furthercomprising maintaining the substrate at a temperature below adecomposition temperature of the zirconium-containing precursor duringdepositing the zirconium oxide film.
 43. The method of claim 42, whereindepositing the zirconium oxide film is performed at a depositiontemperature in a range between 250° C. and 500° C.
 44. The method ofclaim 36, wherein the oxygen-containing precursor is one or moreprecursors selected from the group consisting of an oxide of nitrogen, ahalide-oxygen compound, a peracid, an alkoxide, an alcohol and anoxygen-containing radical.
 45. The method of claim 36, furthercomprising exposing the substrate to pulses of a yttrium-containingmaterial during depositing the zirconium oxide film to form ayttrium-stabilized zirconium oxide film.
 46. The method of claim 36,wherein each pulse of the zirconium-containing precursor deposits aself-saturated layer of zirconium on the substrate.
 47. A method forforming a zirconium oxide film, comprising: exposing a substrate to apulse of a zirconium-containing precursor comprising a plurality ofligands to self-limitingly deposit a zirconium-containing layer on thesubstrate, wherein the plurality of the ligands comprises an amine;subsequently exposing the zirconium layer to a pulse of anoxygen-containing precursor; and sequentially repeating exposing thesubstrate and subsequently exposing the zirconium-containing layer toform a zirconium oxide film of a desired thickness.
 48. The method ofclaim 47, wherein amines constitute a plurality of the ligands.
 49. Themethod of claim 48, wherein the amines are a same amine species.
 50. Themethod of claim 48, wherein the plurality of the ligands comprises amethyl or alkyl group.
 51. The method of claim 50, wherein the pluralityof the ligands comprises a cyclopentadienyl derivative, wherein themethyl or alkyl group is part of the cyclopentadienyl derivative. 52.The method of claim 36, further comprising exposing the substrate topulses of an yttrium-containing material during sequentially repeatingexposing the substrate and subsequently exposing the zirconium layer toform a yttrium-stabilized zirconium oxide film.
 53. The method of claim47, wherein the substrate is a partially fabricated integrated circuit,wherein the zirconium oxide film forms part of a capacitor or atransistor.